AP Biology 2.4 Membrane Permeability Study Notes - New Syllabus Effective 2025
AP Biology 2.4 Membrane Permeability Study Notes- New syllabus
AP Biology 2.4 Membrane Permeability Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Explain how the structure of biological membranes influences selective permeability.
Key Concepts:
- Membrane Permeability
2.4.A.1 – Selective permeability
Phospholipids are major components of the plasma membrane, the outermost layer of animal cells. Like fats, they are composed of fatty acid chains attached to a glycerol backbone. Unlike triglycerides, which have three fatty acids, phospholipids have two fatty acids that help form a diacylglycerol. The third carbon of the glycerol backbone is also occupied by a modified phosphate group. However, just a phosphate group attached to a diacylglycerol does not qualify as a phospholipid. This would be considered a phosphatidate (diacylglycerol 3-phosphate), the precursor to phospholipids. To qualify as a phospholipid, the phosphate group should be modified by an alcohol. Phosphatidylcholine and phosphatidylserine are examples of two important phospholipids that are found in plasma membranes.
Membrane’s Fluidity
A cell’s plasma membrane contain proteins and other lipids (such as cholesterol) within the phospholipid bilayer. Biological membranes remain fluid because of the unsaturated hydrophobic tails, which prevent phospholipid molecules from packing together and forming a solid.
If a drop of phospholipids is placed in water, the phospholipids spontaneously form a structure known as a micelle, with their hydrophilic heads oriented toward the water. Micelles are lipid molecules that arrange themselves in a spherical form in aqueous solution. The formation of a micelle is a response to the amphipathic nature of fatty acids, meaning that they contain both hydrophilic and hydrophobic regions.
2.4.A.2 – Transportation of molecules across membrane
The selective permeability of biological membranes to small molecules allows the cell to control and maintain its internal composition. Only small uncharged molecules can diffuse freely through phospholipid bilayers . Small nonpolar molecules, such as O2 and CO2, are soluble in the lipid bilayer and therefore can readily cross cell membranes. Small uncharged polar molecules, such as H2O, also can diffuse through membranes, but larger uncharged polar molecules, such as glucose, cannot. Charged molecules, such as ions, are unable to diffuse through a phospholipid bilayer regardless of size; even H+ ions cannot cross a lipid bilayer by free diffusion.
Although ions and most polar molecules cannot diffuse across a lipid bilayer, many such molecules (such as glucose) are able to cross cell membranes. These molecules pass across membranes via the action of specific transmembrane proteins, which act as transporters. Such transport proteins determine the selective permeability of cell membranes and thus play a critical role in membrane function. They contain multiple membrane-spanning regions that form a passage through the lipid bilayer, allowing polar or charged molecules to cross the membrane through a protein pore without interacting with the hydrophobic fatty acid chains of the membrane phospholipids.
Channel proteins form open pores through the membrane, allowing the free passage of any molecule of the appropriate size. Ion channels, for example, allow the passage of inorganic ions such as Na+, K+, Ca2+, and Cl- across the plasma membrane. Once open, channel proteins form small pores through which ions of the appropriate size and charge can cross the membrane by free diffusion. The pores formed by these channel proteins are not permanently open; rather, they can be selectively opened and closed in response to extracellular signals, allowing the cell to control the movement of ions across the membrane. Such regulated ion channels have been particularly well studied in nerve and muscle cells, where they mediate the transmission of electrochemical signals.
2.4.A.3 – Nonpolar hydrocarbon tails of phospholipids
A cell’s plasma membrane contain proteins and other lipids (such as cholesterol) within the phospholipid bilayer. Biological membranes remain fluid because of the unsaturated hydrophobic tails, which prevent phospholipid molecules from packing together and forming a solid.
If a drop of phospholipids is placed in water, the phospholipids spontaneously form a structure known as a micelle, with their hydrophilic heads oriented toward the water. Micelles are lipid molecules that arrange themselves in a spherical form in aqueous solution. The formation of a micelle is a response to the amphipathic nature of fatty acids, meaning that they contain both hydrophilic and hydrophobic regions.
2.4.B.1 – Cell walls of Bacteria, Archaea, fungi, and plants
A cell wall is defined as the non-living component, covering the outmost layer of a cell. Its composition varies according to the organism and is permeable in nature. The cell wall separates the interior contents of the cell from the exterior environment. It also provides shape, support, and protection to the cell and its organelles. However, this cellular component is present exclusively in eukaryotic plants, fungi, and a few prokaryotic organisms.
Fungi also possess cell walls, but they are made up of chitin, a derivative of glucose which is also found in the exoskeletons of arthropods. And just like the cell walls in plants, they provide structural support and prevent desiccation.
Prokaryotic organisms such as bacteria also contain cell walls. However, they are chemically different from the cell wall found in plants and fungi. The prokaryotic cell walls are composed of large polymers known as peptidoglycans. Cell walls in prokaryotes serve as a form of protection and prevent lysis (bursting of the cell and expulsion of cellular contents). Structurally, prokaryotic cell walls consist of two layers:
- An inner layer that is made up of peptidoglycans
- An outer layer that is composed of lipoproteins and lipopolysaccharides
Eukaryotic cells possess a definite nucleus along with a distinct nuclear membrane. It also contains membrane-bound organelles not found in prokaryotic cells. Another important point to note is that the cell wall is absent in other eukaryotic organisms such as animals, only plants possess cell walls.
